The invention relates to the field of mass spectrometry, and more particularly to techniques for improved ion separation and filtering. In particular, the invention relates to a mask plate with lobed aperture designed to reduce neutral noise while maintaining the overall intensity of the ion beam.
Mass spectrometers are used today for analyzing various biochemical, organic and biomedical mixtures and compounds. Present limitations to performance (sensitivity and resolution) of quadrupole mass spectrometers are determined by the high mechanical precision needed in the manufacture and alignment of quadrupole mass filters and the limitation of resolution caused by the finite number of R.F. cycles the ions experience when passing through these devices during mass separation. The results often depend on the ion energy and also the deleterious effect of D.C. fringing fields at the entrance of these filters. Particular attention has, therefore, been spent on perfecting instrument components in hopes of improving overall instrument signal and removal of unwanted noise.
Initial studies on improving quadrupole mass filter devices were reported by U. Brinkmann in the International Journal of Mass Spectrometry and Ion Physics. 9 (1972), 161 and were followed by further investigations by A. E. Holmes et al. in the publication International Journal of Mass Spectrometry and Ion Physics, 26 (1978) 191-204. These initial studies sought to simplify the quadrupole electronic circuitry to improve overall signal to noise ratios in the instruments. Since then, due to improvements in electronics and signal detection and analysis, there no longer exist these electrical noise limitations.
Further studies began focusing on simpler and less-expensive mechanical ways for increasing signal and reducing noise in mass spectrometers. For instance, U.S. Pat. No. 4,189,640 of Dawson discloses a quadrupole mass spectrometer having a stop plate employed at the output end of the quadrupole rods to block the passage of axially-directed high-mass particles. However, many of these instruments, instrument parts or improvements have been ineffective in maintaining the overall ion signal. For instance, removing unwanted noise has often involved removing part of the ion beam, blocking part of the beam or interfering with the beam in some other way. These measures impair instrument sensitivity since they result in fewer ions reaching the ion detector. Furthermore, most of the existing mechanical or instrument improvements are unselective in that they simply separate extraneous ions or limit the overall diameter of the ion beam that reaches the detector. For these reasons there is a significant need for a simple device that will remove noise yet substantially maintain the overall signal level of the transmitted ion beam.
More recent work has focused on the classification and determination of the factors that contribute noise to the ion beam, for instance, chemical background noise, electronic noise, and neutral noise. However, removing neutral noise has been quite problematic. Neutral noise is noise apparently caused by excited neutral particles traveling in and near the ion beam path. Neutral noise is believed to be a result of helium metastables, other excited, long-lived species, or energetic photons entering the detector. Improvements have been made by decreasing the size of the quadrupole exit aperture that the ion beam passes through before reaching the detector. However, a side effect of reducing the exit aperture size is eventually an undesirable reduction in the ion signal. In pursuit of improved sensitivity, a goal is to eliminate neutral noise, while at the same time maintaining the level of the ion signal of the compound of interest.
It is, therefore, an object of the invention to provide an improved apparatus and method for reducing neutral noise in a mass spectrometer.
Another object of the invention is to provide an apparatus and method for improving the signal to noise ratio of an ion beam exiting from a quadrupole into a detector.
A still further object of the present invention is to reduce neutral noise in a quadrupolar system while maintaining the intensity of the transmitted ion beam. For example, neutral noise may be reduced, by blocking the particles that cause neutral noise from reaching a detector while at the same time maximizing the signal resulting from the ion beam.
In general, the invention provides a quadrupole mass spectrometer that comprises a mask plate defining a lobed aperture. The mask plate is disposed between a quadrupole mass filter and an ion detector. The lobed aperture has at least one lobe. When first and second lobes are provided, they are symmetric and extend radially from a longitudinal axis. In addition, the lobed aperture may additionally comprise third and fourth symmetric lobes extending radially from a longitudinal axis and orthogonal to the first and second lobes. The mask plate removes neutral atoms that impair the sensitivity of a mass spectrometer with a minimal reduction of the ion beam intensity.